Most irl hunters or gun enthusiasts will know that you can buy most models of a rifle in a few different calibers. I propose this is implemented as a mechanic in Call of the wild. As a "flagship" rifle for this new mechanic I suggest the Ruger No.1, a single-shot breach loader. The Ruger No.1 can be chambered in literally every rifle caliber in call of the wild except .300 blk and .470NE. In total the Ruger No.1 comes in about 72 different calibers. I think this would make for a fine addition as a new rifle and a new mechanic that can be applied to all firearms in the game to some extent, further increasing the players ability to customize and tailor make their own game experience.
How I propose this works;
This mechanic is a free update to the base game. In the base game, the Ruger No.1 is the last rifle to be unlocked, once it's unlocked it can be purchased in any of the base game rifle calibers (including .454, .44 Mag and .357 Mag)
For calibers only accessible via DLC that the Ruger No.1 can use (.375 H&H, .30-06, .308 etc.) I propose that those calibers only be shown as a purchasable option for the rifle assuming the player has the required DLC to purchase and use that caliber